Kawakami Y, Muraoka Y, Kubo K, Suzuki Y, Fukunaga T

J. Gravitat. Physiol, 7: 53-60, 2000

Five healthy men carried out a program of head-down bed rest (BR) for 20 days. Before and after BR, a series of cross-sectional scans of the thigh were performed using magnetic resonance imaging, from which volumes of the quadriceps muscles were determined and physiological cross-sectional areas (PCSA) were calculated. Muscle thickness and pennation angles of the triceps brachii, vastus lateralis, and triceps surae muscles were also determined by ultrasonography. During BR, subjects performed unilateral isokinetic knee extension exercises every day. The contralateral limb served as a control. Decrease in PCSA after BR was greater in the control (-10.2 +- 6.3 %) than in the trained limb (-5.2 +- 4.2 %) in all subjects. Among the quadriceps, vastus intermedius in the control limb was predominantly affected by BR with respect to the volume and PCSA. Thickness of the lateral gastrocnemius and soleus showed a larger decrease than that of the triceps brachii and vastus lateralis, while neither trained nor control muscles showed significant changes in pennation angles after BR. The results suggest that the effects of bed rest and exercise during bed rest is muscle-specific, and that muscle pennation is not notably affected by unloading for up to 20 days.